NAME

       img2grd - Extract region of img in Mercator or geographic form

SYNOPSIS

       img2grd imgfile -Ggrdfile -Rwest/east/south/north[r] -Ttype  [ -C ] [ -D[minlat/maxlat] ] [ -E ] [ -L ] [
       -M ] [ -Nnavg ] [ -Sscale ] [ -V ] [ -Wmaxlon ] [ -mminutes ]

DESCRIPTION

       img2grd is a front-end to img2mercgrd which reads an img format file and creates a  grid  file.   The  -M
       option dictates whether or not the Spherical Mercator projection of the img file is preserved.

       imgfile
              An  img  format  file  such  as  the  marine  gravity or seafloor topography fields estimated from
              satellite altimeter data by Sandwell and Smith.  If the  user  has  set  an  environment  variable
              $GMT_DATADIR,  then img2mercgrd will try to find imgfile in $GMT_DATADIR; else it will try to open
              imgfile directly.

       -G     grdfile is the name of the output grid file.

       -R     west, east, south, and north specify the Region of interest, and you may specify them  in  decimal
              degrees  or  in  [+-]dd:mm[:ss.xxx][W|E|S|N]  format.   Append r if lower left and upper right map
              coordinates are given instead of w/e/s/n.  The two shorthands -Rg and -Rd stand for global  domain
              (0/360  and -180/+180 in longitude respectively, with -90/+90 in latitude). Alternatively, specify
              the name of an existing grid file and the -R settings (and grid spacing, if applicable) are copied
              from the grid.

       -T     type  handles the encoding of constraint information.  type = 0 indicates that no such information
              is encoded in the img file (used for pre-1995 versions of the gravity data)  and  gets  all  data.
              type  >  0  indicates that constraint information is encoded (1995 and later (current) versions of
              the img files) so that one may produce a grid file as follows: -T1 gets data values at all points,
              -T2  gets  data  values  at  constrained  points  and  NaN  at  interpolated points; -T3 gets 1 at
              constrained points and 0 at interpolated points.

OPTIONS

       -C     Set the x and y Mercator coordinates relative to projection center [Default is relative  to  lower
              left corner of grid].  Requires -M.

       -D     Use  the  extended  latitude  range  -80.738/+80.738.   Alternatively, append minlat/maxlat as the
              latitude extent of the input img file.  [Default is -72.006/72.006].

       -E     Can be used when -M is not set to force the final grid to have the exact same region as  requested
              with  -R.  By default, the final region is a direct projection of the original Mercator region and
              will typically extend slightly beyond the requested  latitude  range,  and  furthermore  the  grid
              increment  in  latitude  does  not  match  the longitude increment.  However, the extra resampling
              introduces small interpolation errors and should only be used if the output grid  must  match  the
              requested  region  and  have  x_inc  =  y_inc.  In this case the region set by -R must be given in
              multiples of the increment (.e.g, -R 0/45/45/72).

       -L     With no other arguments, list all *.img files found in the directory pointed to  by  $GMT_DATADIR,
              or  the  current  directory  if  not defined.  Ignored if other options are present on the command
              line.

       -M     Output a Spherical Mercator grid [Default is a geographic lon/lat grid].

       -N     Average the values in the input img pixels into navg by navg squares, and create one output  pixel
              for  each such square.  If used with -T3 it will report an average constraint between 0 and 1.  If
              used with -T2 the output will be average data value or NaN according to whether average constraint
              is  > 0.5.  navg must evenly divide into the dimensions of the imgfile in pixels.  [Default 1 does
              no averaging].

       -S     Multiply the img file values by scale before storing in grid file.  [Default is 1.0].  For  recent
              img  files:  img  topo  files  are  stored in (corrected) meters [-S 1]; free-air gravity files in
              mGal*10 [-S 0.1 to get mGal]; vertical   deflection  files  in  microradians*10  [-S  0.1  to  get
              microradians], vertical gravity gradient files in Eotvos*50 [-S 0.02 to get Eotvos, or -S 0.002 to
              get mGal/km]).

       -V     Selects verbose mode, which will send  progress  reports  to  stderr  [Default  runs  "silently"].
              Particularly recommended here, as it is helpful to see how the coordinates are adjusted.

       -m     Indicate minutes as the width of an input img pixel in minutes of longitude.  [Default is 2.0].

       -W     Indicate  maxlon  as the maximum longitude extent of the input img file.  Versions since 1995 have
              had maxlon = 360.0, while some earlier files had maxlon = 390.0.  [Default is 360.0].

EXAMPLES

       To extract data in  the  region  -R-40/40/-70/-30  from  world_grav.img.7.2  and  preserve  the  Mercator
       gridding:

       img2grd world_grav.img.7.2 -G merc_grav.grd -R-40/40/-70/-30 -M -T 1 -V

       Without the -M option the same command will yield a geographic grid.

SEE ALSO

       GMT(1), img2mercgrd(1)